Abstract
Three iridoviruses have been identified in Florida fisheries over the past 10 yr. Although PCR on the three viruses has not been done at this time, the three agents respond differently to in vitro handling, suggesting that they are three separate entities. The first, largemouth bass iridovirus, was detected in the early 1990s in free-ranging largemouth bass from Lake Weir, Florida. The virus was detected by inoculating cell cultures, but was not associated with clinical disease. The virus was isolated using Fat Head Minnow (FHM) and Tilapia Heart (TMB) cell lines and maintained at the Department of Agriculture and Consumer Services (DOACS) veterinary diagnostic laboratory in Kissimmee, Florida. A serologic test was developed using an agar-gel diffusion technique, and a pilot study run, testing four populations of largemouth bass for exposure to the agent. Of 30 bass tested in each of four lakes, 53% were seropositive from Lake Weir, 73% were seropositive from Lake Holly, 53% were seropositive from Lake Harris and 70% were seropositive from Lake Newnan.2 Of the four lakes tested, Lake Holly had the highest incidence of seropositive fish. This lake had been included in the study as a control site because of its relatively pristine condition and the lack of obvious health problems in resident fish, despite frequent sampling. Preliminary work with largemouth bass iridovirus in Florida suggested that the agent was non-pathogenic to Florida largemouth bass, although more recent observations in other states have linked the agent with fish kill events in free-ranging fish.4
In the 1990s an iridovirus was detected on several commercial ornamental fish farms in central Florida. The agent was associated with clinical disease in freshwater angelfish, Pterophyllum scalare, and swordtails, Xiphophorus helleri. The agent has not been successfully isolated in tissue culture and therefore River's postulates have not been tested. The clinical assessment at this time is that the agent is responsible for clinical disease and economic loss. Schuh and Shirley5 have previously implicated an iridovirus with mortality of cultured freshwater angelfish. Additional work on the iridovirus detected in Florida angelfish is in progress.
An iridovirus has also been described in gouramis of the family Trichogaster.1 The agent has been isolated using the TMB cell line. The gourami agent will not grow in FHM cells, and it does not produce as much cytopathic effect as the largemouth bass agent on TMB cells. These observations suggest that it is distinct from the largemouth bass agent. The gourami agent has been associated with clinical disease and is believed to be pathogenic at water temperatures of 32°C. A study was completed in which River's postulates were tested, and results, though not conclusive, strongly supported the hypothesis that the gourami iridovirus was pathogenic.3 Characteristics of the three agents will be reviewed as well as clinical significance of each, given the current state of knowledge.
References
1. Fraser WA, TJ Keefe, B Bolon. 1993. Isolation of an iridovirus from farm-raised gouramis (Trichogaster tricopterus) with fatal disease. J. Vet. Diag. Invest. 5:250-253.
2. Haworth M. 1993. Prevalence of an unknown iridovirus in largemouth bass (Micropterus salmoides) in four central Florida lakes. Senior Research Project, University of Florida College of Veterinary Medicine, Gainesville, FL, Feb 25, 1993.
3. Lynch MJ. 1998. Preliminary investigation of gourami iridovirus infections. Master's Thesis, University of Florida, Department of Fisheries and Aquatic Sciences. August, 1998.
4. Plumb JA, JM Grizzle, HE Young, AD Noyes. 1996. An iridovirus isolated from wild largemouth bass. J. Aquatic Anim. Health 8:265-270.
5. Schuh JCL, IG Shirley. 1990. Viral hematopoietic necrosis in an angelfish (Pterophyllum scalare). J. Zoo Wildl. Med. 21(1):95-98.